It is likely that a wireless backhaul system for small cells operates in a non-line-of-sight (NLOS) environment. Conventionally, most wireless backhaul systems are designed for microwave communications in a line-of-sight (LOS) environment which guarantees a high availability of a desired data rate over a communication link or link, respectively. LOS operation also guarantees an extremely low outage probability, i.e. the link is in principle never in outage which is important when designing wireless backhaul systems that transport user data and control data traffic between radio base stations (RBSs).
It is desirable that a wireless backhaul system operating in NLOS should also guarantee a high availability and thus a very low outage probability. It is however difficult to achieve this when using microwave frequencies, for instance microwave frequencies larger than 10 GHz, for backhauling in NLOS. Nowadays a general consensus within the wireless industry is that the capacity offered in existing cellular radio access networks based on macro cells cannot fulfil the requirements for future mobile broadband services unless a capacity boost is achieved at specific locations, such as at hotspots, cell edges and indoor locations. One approach to increase the capacity at these locations is to deploy low power radio base stations, such as pico RBSs, covering smaller cells within the macro cell coverage area in a heterogeneous network. Assuming that each macro cell is supported by a few small cells, the number of cells in the network and thus the required number of mobile backhaul (MBH) links will increase dramatically. Due to its small coverage area it will be important that the small cell pico RBS is arranged in a correct location, not limited by the availability of a broadband connection for MBH. Further, the pico RBS will in many cases be placed or arranged below rooftop level of a building, preventing a clear LOS between the pico RBS and an aggregation node that is co-located with a macro RBS above rooftop. Typically, the macro RBS is arranged on a rooftop of a building. The macro RBS is characterized by a large coverage area, by high power and directional antennas. The pico RBS is typically arranged up to 6 meters above street level, for instance on a building wall, dependent on the desired application and environment. In an NLOS environment communication via a backhaul channel is primarily based on diffraction and/or reflection. Traditional MBH technologies, such as copper, optical fiber or LOS microwave links, may not always fit to such a heterogeneous backhaul scenario and thus they need to be complemented by low cost wireless NLOS MBH links. NLOS propagation has traditionally been proposed only for carrier frequencies below 6 GHz. However, wideband spectrum on these frequencies is a scarce resource and if made available it becomes attractive to utilize this spectrum for mobile broadband services in a radio access network (RAN).
Traditional fixed service LOS microwave point-to-point (PtP) links operate in licensed frequency bands between 6 GHz and 42 GHz on channel bandwidths ranging from 3.5 MHz up to 112 MHz. Due to the nature of the traffic carried by the MBH, traditional PtP LOS links are required to have an extremely high availability. However, if a small cell is within the coverage area of a macro cell in a heterogeneous deployment, an extremely high availability requirement on the small cell backhaul can be debated. The requirement for a small cell MBH is becoming an increasingly important topic that is discussed within the industry and it is driven by the evolution towards flexible and cost effective heterogeneous deployments.
Over the last few years there has been an increased interest for the higher frequency bands for MBH applications, in particular for the 60 GHz band comprising a 9 GHz of bandwidth between 57 to 66 GHz and for the 70/80 GHz band comprising a total of 10 GHz bandwidth between 71 to 86 GHz. Compared to the 56 and 112 MHz channels available for traditional microwave links, the bandwidths offered in these bands are large which makes them attractive. Parts of the 60 GHz band is license free spectrum which means that anyone can deploy systems within that band as long as the equipment complies with predefined regulations. Nevertheless, this frequency is particularly interesting for small cell MBH applications since the excess oxygen loss at this band may attenuate interference from neighbouring links enabling efficient frequency reuse.
In cellular communications a mobile user is associated or connected to a cell. The choice of a cell is usually based on being connected to the, in some sense, best cell for the particular user. Due to mobility of a user, the user might experience another cell as an alternative connection. If needed, the system may handover the user to another cell. Conventionally, handovers are based on measurements on the access links from different cells or base stations.
A microwave-based wireless backhaul system operating in NLOS will have problems with guaranteeing a high availability and low outage probability due to a number of issues related to microwave propagation in urban NLOS. Some typical disadvantages and problems are described in the following: Weather effects, such as rain and snow, will change the diffraction properties; non-existing penetration of obstacles since high-frequency radio waves rely mostly on reflections and diffractions; trees and other objects may move in the wind; temporary obstacles, for instance tall vehicles passing close to the small cell RBS mounted at or just above street level; long-term changes in the urban topology, for instance new buildings, commercial billboard signs and so on; unexpected multipath propagation due to strong reflections; and high gain antennas, i.e. pencil beam antennas, that become misaligned due to weather effects or vandalism.
There is a need to provide high gain antennas for compensating for the high path loss associated with high frequencies in an NLOS environment. If the backhaul to/from a particular access point fails or is lost, the users connected to that particular access point could ideally do a handover to another access point with a functioning backhaul. However, a failed backhaul connection means that the network has lost all of its communication to/from that access point which makes it impossible for the access point to request such a handover of its users.
Instead, this handover has to be requested by the users themselves which might mean that many users have to quickly do simultaneous handover requests which increase the likelihood of dropped connections. It could be that the users might have a poor, a very poor or even a non-existing connection to an alternative access point. For instance, the users might be indoors and connected to an indoor access point that receives its backhaul connection via an outdoor or close to an outdoor antenna solution.